After only six months in orbit about Mercury, a NASA spacecraft has collected measurements that have discredited most theories about how our solar system’s innermost planet formed. Data gathered by instruments on MESSENGER reveal that Mercury’s surface has Earth-like levels of potassium and an even higher sulfur abundance, evidence that is at odds with most theories for how the super-dense planet came to be.

Launched in August 2004, MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It entered orbit about Mercury—the first spacecraft to do so—in March of this year.

Some of these new findings, published in a set of seven Science papers available online today, were first predicted in 2003 by Denton Ebel, curator in the Museum’s Department of Earth and Planetary Sciences, and Conel Alexander, a researcher in the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. The two scientists recently published a separate paper highlighting their model in Planetary and Space Science.

“The composition of Mercury is very different from that of the Moon and other terrestrial planets,” said Ebel, who is a co-author on two of the new Science papers and the science lead for MESSENGER education and outreach efforts at the Museum.

The most striking difference between Mercury and other planets is the size of Mercury’s iron core, which makes up about 65 percent of the planet’s total mass. Earth’s core, by comparison, is just 32 percent of its mass. This characteristic is especially curious because the terrestrial planets—Mercury, Venus, Earth, and Mars—share similar early histories, each forming from accumulated dust, ice, and gas in the early solar nebula.

What explains Mercury’s proportionally larger core? Until now, one theory held that the Sun’s heat, which warms Mercury to nearly 450 degrees Celsius, or 840 degrees Fahrenheit, on its sunny side, vaporized Mercury’s outer layers of rock, leaving behind a heavier core. But findings that confirm Mercury’s high-potassium, high-sulfur surface discount that possibility.

“You would think that kind of heat would have boiled off potassium and sulfur, which are very volatile and vaporize at relatively low temperatures,” Ebel said. “Yet we see high ratios of those elements compared to the ones that don’t evaporate so quickly. This is not the reason why Mercury’s mantle is so small, relative to the core.”

MESSENGER’s compositional analysis also complicates other competing hypotheses: that a major impact stripped off much of the planet’s original mantle, or that the innermost regions of the early solar nebula became enriched with large iron-rich, boulder-sized bodies, leading to Mercury’s metal-heavy composition.

“Right away, we’re learning things that allow us to eliminate theories of the origin of Mercury,” Ebel said.

There’s one prediction that is supported by MESSENGER’s findings. In their latest paper, Ebel and Alexander propose that Mercury’s formation zone was enriched in carbon-rich interplanetary dust that caused sulfur to condense into solid, less volatile minerals at very high temperatures. This scenario accounts for the planet’s high levels of sulfur, although not for its hefty iron core. For the latter consequence, it’s back to the drawing board.

“Every time we get an up-close look at a planet, we learn a lot more about how planets form,” said Ebel. “Planet formation is a puzzle humans have been trying to solve for hundreds of years, and we’re finally beginning to get a hold of it.”

Click here for more news about MESSENGER from the Johns Hopkins University Applied Physics Laboratory.

To watch a feature about MESSENGER from the Museum’s Science Bulletins, click here.